The present invention relates to techniques for preventing circuit malfunctions to take reliable safety measures in a discharge lamp lighting circuit which is configured to supply the power to a discharge lamp and a control circuit when switching means arranged on respective power supply paths, which are provided to receive two or more power supply lines, are switched on in synchronism with one another.
As a lighting circuit for a discharge lamp (metal halide lamp or the like), a configuration comprising a direct current power supply circuit, a direct current-to-alternating current converter circuit, and a starter circuit (so called a starter circuit) is known.
For example, the lighting circuit is supplied with the power from a direct current power source through an over-current protecting element (a fuse or the like) and a lighting switch, and a control circuit is provided for controlling the power supplied to a discharge lamp. A power supply voltage supplied to the control circuit may be the same voltage as the direct current voltage inputted to the lighting circuit, used as it is, or may be produced from the direct current voltage by a regulated voltage power supply circuit.
When a discharge lamp experiences a failure in lighting or when an input voltage to a lighting circuit presents an abnormal value, the operation of the lighting circuit is stopped to prevent safety hazard to a human body due to a high voltage or to obviate damages such as fuming, firing and so on resulting from an excessive power output.
However, in a circuit which is configured such that a control circuit receives the same power supply input as alighting circuit for operation, when the power supply from a direct current power supply is interrupted by an over-current protecting element, the power supply to the control circuit is also stopped, thereby failing to take sufficient safety measures (for example, lighting a substitute light source, alarming the occurrence of a failure, and so on).
It is therefore contemplated to provide a switch means on each of power supply paths such that the power supply can be received on two or more lines, so that a discharge lamp and a control circuit are both supplied with the power when the respective switch means are switched on in synchronism with each other. Specifically, even if a failure occurs on a power supply path to a lighting circuit to interrupt the power supply to the lighting circuit, the operation of the control circuit is ensured as long as the power supply path to the control circuit is normal, thereby making it possible to detect a failure occurring in the discharge lamp or the lighting circuit by the control circuit to take safety measures such as lighting another light source as a substitute light source for the discharging lamp, alarming the occurrence of a failure, and so on.
However, since it is difficult to switch on/off the respective switch means provided on the respective power supply paths completely in synchronism with each other (specifically, a plurality of elements, even in the same specifications, cannot be switched on simultaneously at a correct timing due to a shift in on-timing caused by dimensional errors and variations in the manufacturing process of the switching means, and delayed operations due to response speeds of the switching elements), there is a problem in that the circuit is likely to suffer from malfunctions.
It is therefore an object of the present invention to take sufficient safety measures in a lighting circuit which comprises switching means provided on each of power supply paths such that power supplies can be received on two or more lines, and is configured to supply the power to a discharge lamp and a control circuit when the switch means are switched on substantially in synchronism with each other, by preventing malfunctions from occurring due to a shift in open/close timing of each switch means.
To achieve the above object, a discharge lamp lighting circuit comprises: a lighting circuit which receives a power supply from a power source and lights on a discharge lamp; a control circuit which detects a failure occurring in a discharge lamp or the lighting circuit so that the power supply to the discharge lamp or operation of the lighting circuit is stopped, and another light source as a substitute light source for the discharge light is lighted, or the occurrence of the failure is notified; at least two switching means, one being connected to said lighting circuit for receiving power supply, while the other being connected to said control circuit for receiving another power supply, the respective switches being switched on substantially in synchronism with one anther; wherein said control circuit is prohibited from stopping the power supply to the discharge lamp, or stopping the operation of the lighting circuit, or lighting the substitute light source, or notifying the occurrence of a failure until a predefined time period is elapsed from the time said switch means are switched on to cause the lighting circuit to start operating.
Therefore, according to the present invention, since the control circuit is prohibited from stopping the power supply to the discharge lamp or stopping the operation of the lighting circuit, or lighting the substitute light source, or notifying the occurrence of a failure until the predefined time period is elapsed after the lighting circuit has started the operation, it is possible to prevent malfunctions of the control circuit due to a shift in open/close timing of the switch means.